Fuselage, Empennage & Trim Tab

The fuselage is an aircraft's main body section that holds crew and passengers or cargo.

Empennage is an aviation term used to describe the tail portion of an aircraft.

Trim tabs are small surfaces connected to the trailing edge of a larger control surface on a boat or aircraft. The angle of the tab relative to the larger surface can be adjusted to null out hydro- or aero-dynamic forces and stabilise the boat or aircraft in a particular desired attitude without the need for the pilot to constantly apply a control force.

Fuselage of a Boeing 737

The fuselage can be short, and seemingly unaerodynamic, as in this Christen Eagle 2

The fuselage (from the French fuselé "spindle-shaped") is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull. The fuselage also serves to position control and stabilization surfaces in specific relationships to lifting surfaces, required for aircraft stability and maneuverability.

Types of structures

Box truss structure

The structural elements resemble those of a bridge, with emphasis on using linked triangular elements. The aerodyamic shape is completed by additional elements called formers and stringers and is then covered with fabric and painted. Most early aircraft used this technique with wood and wire trusses and this type of structure is still in use in many lightweight aircraft using welded steel tube trusses. This method is especially suitable for amateur-built aircraft kits, where a complete welded truss structure is delivered with the fitting of other components, covering, and finishing completed by the user, as it ensures that a robust, uniform load bearing structure is within the completed aircraft.

Geodetic construction

Geodetic structural elements were used by Barnes Wallis for British Vickers between the wars and into World War II to form the whole of the fuselage, including its aerodynamic shape. In this type of construction multiple flat strip stringers are wound about the formers in opposite spiral directions, forming a basket-like appearance. This proved to be light, strong, and rigid and had the advantage of being made almost entirely of wood. The structure is also redundant and so can survive localized damage without catastrophic failure. A fabric covering over the structure completed the aerodynamic shell (see the Vickers Wellington for an example of a large warplane which uses this process). The logical evolution of this is the creation of fuselages using molded plywood, in which multiple sheets are laid with the grain in differing directions to give the monocoque type below.

The Vans RV-7 fuselage is slender for high speed flight

Monocoque shell

In this method, the exterior surface of the fuselage is also the primary structure. A typical early form of this (see the Lockheed Vega) was built using molded plywood, where the layers of plywood are formed over a "plug" or within a mold. A later form of this structure uses fiberglass cloth impregnated with polyester or epoxy resin, instead of plywood, as the skin. A simple form of this used in some amateur-built aircraft uses rigid expanded foam plastic as the core, with a fiberglass covering, eliminating the necessity of fabricating molds, but requiring more effort in finishing (see the Rutan VariEze). An example of a larger molded plywood aircraft is the de Havilland Mosquito fighter/light bomber of World War II. It should be noted that no plywood-skin fuselage is truly monocoque, since stiffening elements are incorporated into the structure to carry concentrated loads that would otherwise buckle the thin skin. The use of molded fiberglass using negative ("female") molds (which give a nearly finished product) is prevalent in the series production of many modern sailplanes. The use of molded composites for fuselage structures is being extended to large passenger aircraft such as the Boeing 787 Dreamliner (using pressure-molding on female molds).

Semi-monocoque

Sectioned fuselage of a Boeing 747 showing frames, stringers and skin all made out of Aluminium

This is the preferred method of constructing an all-aluminum fuselage. First, a series of frames in the shape of the fuselage cross sections are held in position on a rigid fixture, or jig. These frames are then joined with lightweight longitudinal elements called stringers. These are in turn covered with a skin of sheet aluminum, attached by riveting or by bonding with special adhesives. The fixture is then disassembled and removed from the completed fuselage shell, which is then fitted out with wiring, controls, and interior equipment such as seats and luggage bins. Most modern large aircraft are built using this technique, but use several large sections constructed in this fashion which are then joined with fasteners to form the complete fuselage. As the accuracy of the final product is determined largely by the costly fixture, this form is suitable for series production, where a large number of identical aircraft are to be produced. Early examples of this type include the Douglas Aircraft DC-2 and DC-3 civil aircraft and the Boeing B-17 Flying Fortress. Most metal light aircraft are constructed using this process.

Both monocoque and semi-monocoque are referred to as "stressed skin" structures as all or a portion of the external load (i.e. from wings and empennage, and from discrete masses such as the engine) is taken by the surface covering. In addition, all the load from internal pressurization is carried (as skin tension) by the external skin.

Exceptions

Conversely there have been a small number of aircraft designs which have no separate wing, but use the fuselage to generate lift. Examples include NASA's experimental lifting body designs and the Vought XF5U-1 Flying Flapjack.

See also

External links

Empennage of a Cessna 172

Empennage [ émpənij ] is an aviation term used to describe the tail portion of an aircraft. ("Empennage", "tail", and "tail assembly" may be interchangeably used.) The empennage gives stability to the aircraft and controls the flight dynamics: pitch and yaw. In simple terms the empennage may be compared to the feathers of an arrow, colloquially; "Tail Feathers"

Structurally, the empennage consists of the entire tail assembly, including the fin, tailplane and the part of the fuselage to which these are attached. On an airliner this would be everything behind the rear pressure bulkhead.

The front, usually fixed section of the tailplane is called the horizontal stabilizer and is used to balance and share lifting loads of the mainplane dependent on centre of gravity considerations by limiting oscillations in pitch. The rear section is called the elevator and is usually hinged to the horizontal stabilizer. The elevator is a movable airfoil that controls changes in pitch, the up-and-down motion of the aircraft's nose.

On some aircraft, the horizontal stabilizer and elevator are combined into one movable unit called the stabilator or sometimes "flying tail" (see Anderson, John D., Introduction to Flight, 5th ed, p 517). In all cases some arrangement is made for the provision of trim to allow minor adjustment of airflow over the control surface and to unload the pilot from the need to maintain constant pressure on the elevator control. The trim may take the form of trim tabs on the rear of the elevators which act to force the elevator in the desired direction, or the stabilizer may be hinged at its trailing edge, forward of the elevator and adjustably jacked a few degrees in incidence either up or down. Early aircraft had a spring in the control circuit which provided an adjustable preload in the desired direction.

The vertical tail structure, or fin, also has a fixed front section called the vertical stabilizer, used to prevent the aircraft from yawing from side to side. The rear section of the vertical fin is the rudder, a movable airfoil that is used to turn the aircraft in combination with the ailerons.

Occasionally the horizontal stabilizer may carry more than one fin and rudder (Avro Lancaster, Lockheed Constellation) or the stabilizer and fin may be combined into a "V" shaped structure (Ruddervators) with each of the angled airfoils performing both functions (Beechcraft Bonanza 35, Fouga Magister). Frequently the horizontal stabilizer is mounted atop the fin (Boeing 727, Piper Tomahawk) Additional fin area may be added to aircraft fitted with floats (seaplanes) usually beneath the horizontal stabilizer (ventral fin) and sometimes at the stabilizer extremities.

Multi engined and some light aircraft also include trim tabs on the rudder when asymmetric forces would impose unusual loads on the pilot's rudder controls.

See also

The center console of a small airplane. The vertical black wheel with spherical bumps is the trim-tab control. Moving it upwards lowers the airplane nose and increases speed; moving it downwards raises the airplane nose and reduces speed.

When mounted on a control surface (such as an elevator or rudder), the trim tab allows the pilot to easily adjust the position of the control surface, and to adjust the large force generated by the control surface. The trim tab acts as a servo tab. Because the center of pressure of the trim tab is further away from the axis of rotation of the control surface than the center of pressure of the control surface, the moment generated by the tab can match the moment generated by the control surface. The position of the control surface on its axis will change until the moments from the control surface and the trim surface balance each other.

Uses in boats

Boats with planing-type hulls will often have trim tabs attached to the trailing edge of the hull or transom. These are used to adjust the pitch attitude of the boat while underway. Changes in boatspeed or weight placement will usually require the trim tabs to be adjusted to keep the boat at a comfortable and efficient pitch attitude. This reduces the work of the captain by reducing the amount of manual control necessary, as well as providing for greater efficiency by keeping the ship in the ideal orientation for the conditions.

Trim Tabs are most found on cruisers, sport fishing boats and center console boats ranging from 20 feet and up.

In some sailboats, the trailing edge of the keel has a trim tab which is used to null out rudder forces (lee or weather helm).

Sailboats intended for long-distance voyages with minimum crew often have a self-steering system which works via a trim tab (usually called an anti-servo tab) on the trailing edge of the rudder. This tab is mechanically connected to a wind vane. The system automatically adjusts the boat's heading to keep it at a fixed angle of attack to the apparent wind.

Uses in aircraft

Many airplanes (including gliders) have trim tabs on their elevators, this being a simple method of providing trim in the lateral axis.

All aircraft must have a system for ensuring trim in the lateral axis, sometimes employing more cunning and sophisticated methods. Alternatives to trim tabs include:

Elevator trim frees the pilot from constantly adjusting the pitch controls. A longitudinal trim control (often in the shape of a wheel) is adjusted by the pilot to cancel out control forces for a given airspeed or weight distribution. When the trim control is rotated forward the nose is held down and conversely if the trim wheel is moved back the tail becomes heavy. Many newer aircraft, especially jet aircraft have electric trim controls.

Many airplanes also have rudder and/or aileron trim systems. Some aircraft have a rudder trim tab that is rigid but adjustable by bending on the ground. It is angled slightly to the left to lessen the need for the pilot to push the rudder pedal constantly to overcome the left-turning tendencies of some prop-driven aircraft. Many aircraft also have control wheels inside the cockpit so the pilot can adjust a hinged rudder trim while in flight.

When a trim tab is employed, it is moved into the slipstream opposite to the control surface's desired deflection. For example, in order to trim an elevator to hold the nose down, the elevator's trim tab will actually rise up into the slipstream. The increased pressure on top of the trim tab surface caused by raising it will then deflect the entire elevator slab down slightly, causing the tail to rise and the aircraft's nose to move down.^[1] In the case of an aircraft where deployment of high lift devices(flaps) would significantly alter the longitudinal trim, a supplementary trim tab is arranged to simultaneously deploy with the flaps so that pitch attitude is not markedly changed.

The use of trim tabs significantly reduces the workload on pilots, since their attention can be focused on tasks other than control input during continuous manoeuvres (ie: sustained climb on takeoff to high altitude, descent prior to landing), such as traffic avoidance, or communication with air traffic control.

Not only does keeping an aircraft trimmed properly reduce pilot workload, it also increases fuel efficiency by reducing drag. For example, if an aircraft is climbing it may have a tendency to yaw which increases parasite drag because the craft is not flying straight into the flight path. The yaw may be able to be reduced by use of the rudder trim tab.

Trim Tab as Metaphor

The engineer Buckminster Fuller is often cited for his use of trim tabs as a metaphor for leadership and personal empowerment. In the February 1972 issue of Playboy, Fuller said:

References

^ The Anatomy Of The Aeroplane Darrol Stinton, ISBN 0-632-01876-3

Contents

Contents

Types of structures

Box truss structure

Geodetic construction

Monocoque shell

Semi-monocoque

Exceptions

See also

External links

See also

Contents

Uses in boats

Uses in aircraft

Trim Tab as Metaphor

References